The position of MSI capability is already cached in the msi_desc when
we enter the msi_set_mask_bits().  Use it instead.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>